In a switching power supply, an alternating current (AC) power signal delivered from a source may be full wave rectified by a full wave bridge to create a direct current (DC) signal having a ripple at twice the frequency of the source. The full wave rectified signal may be referred to as the input voltage Virect, and may couple to a first terminal of a boost inductor. The second terminal of the boost inductor of the boost circuit may couple to a boost switch and a boost diode. A boost circuit, which may comprise the boost inductor boost switch and boost diode, may have two distinct phases of operation: an inductor charge cycle, and an inductor discharge cycle.
During the inductor charge cycle, the boost switch may be closed (placed in a conductive state) thereby allowing electrical current to flow through the boost inductor, the boost switch, and then to ground. As the current through the boost inductor increases, energy may be stored in the magnetic field of the boost inductor. When sufficient energy is stored in the boost inductor, the boost switch may open, and the second phase of operation, the inductor discharge cycle, may begin. The collapsing magnetic field of the boost inductor in the discharge cycle may create a voltage which forward-biases the boost diode, allowing current to flow through the boost diode to a load RL.
Although it may be possible to utilize a boost circuit without regard to power factor of a source, it may be desirable that the electrical current waveform drawn from the source substantially match the voltage waveform of the source—power factor control. Having the electrical current drawn from the source match the voltage waveform may be accomplished by pulse-width modulating a control signal applied to the boost switch. As the source voltage increases or decreases, so too does the current draw. To accomplish this, control changes may be made based on source voltage. The output voltage too may need to be monitored and controlled. Thus, there may be two distinct control loops for control of a boost circuit—a power factor control loop (which may also be referred to as a current loop) and a voltage output control loop.
With regard to power factor or current loop control, this first “loop” may involve a comparison of the input voltage Virect to the total input current Ii (including inductor charge current not supplied to the load). Signals representing the input voltage and the current may be subtracted to create an error signal. The current control loop may be a very fast loop, meaning that control changes may be made at a frequency significantly higher than a ripple frequency in the rectified source waveform. In order to achieve the power factor correction in continuous current mode systems, adjustments to the duty cycle of the pulse width modulated waveform may be made at approximately the same frequency as the switching frequency. Continuous current mode systems may not allow boost inductor current to drop to zero before alternating to an inductor charge cycle, which may again increase current flow. By contrast, systems that operate in discontinuous current mode may be designed to allow boost inductor current to drop to zero before again entering an inductor charge cycle. With the switching frequency ranging from 50 to 100 kilohertz, corrections applied by the current loop may take place at the same frequency. The signal generated by the current loop may be applied to a pulse width modulator, which in turn may create and modify the duty cycle of the signal applied to the boost switch.
The second controlled parameter in a boost circuit may be the DC output voltage V0. Since the controllable variables may be the frequency and duty cycle of the signal applied to the boost switch 10, there may be overlap of control between the current loop previously discussed and the voltage loop. The voltage control loop may comprise a reference voltage VREF, which may represent the desired output voltage summed with the actual output voltage V0. So that the voltage control loop does not become unstable due to the interaction of the two control loops, it may be necessary that the loop be sufficiently slow, or have a low bandwidth, that reactions to the ripple in the output voltage related to the ripple (AC component) of the input voltage may not be made instantaneously. Stated otherwise, if the voltage loop attempts to apply corrections near the frequency of the ripple current, the loop may become unstable. In order to address this factor, voltage control loops may have limited bandwidth and therefore control tolerances for output voltage during load transient conditions may be very wide. In analog systems, a limited bandwidth may be accomplished by a low pass filter coupled between the sensed output voltage V0 and a circuit where the reference signal and the sensed output voltage are summed. The low pass filter, as the name implies, may allow only the lower frequency signals to pass, and the signals passed may be below ripple frequency. The summation of the reference signal and the low pass filtered output voltage may create an error signal which may be applied to proportional and integral components of the control loop. The output of the voltage control loop may be summed with an output of an input current control loop to create the signal to the pulse width modulation system.
Analyzing the two loops in parallel, the current loop may make the primary determination as to the duty cycle of the pulse width modulated signal for power factor correction purposes, and the voltage loop, on a much slower basis, may make corrections to maintain the set point output voltage. Again, the limited bandwidth voltage control loop may lead to large output voltage swings, especially in transient conditions.
The control loops described may be implemented in analog format with control parameters (e.g. loop gain) fixed by resistors and capacitors. Component tolerances and variations in manufacturing, as well as aging of the circuit components, may result in changes over time to the control loop response. The control loops described may likewise be implemented in digital systems, but this may merely implement the analog control loops in digital form.
Implementation of control loops for switching power supplies, whether in analog or digital format, may require measuring total input current, which may comprise electrical current supplied to the load and boost inductor charge current (which may not be supplied to the load). Measuring current may be difficult and/or require significant space within the power supply. Further still, the analog control systems may assume a sinusoidal input waveform, and thus line disturbances that may affect the sinusoidal character of the input voltage may adversely affect the voltage output control loops.
Some switching power supplies may implement multiple boost circuits, each switched slightly out of place with the others. While switching power supplies with multiple boost circuits may be useful for reducing ripple in the output voltage, their advantage may be lost when the power supply is in a lightly loaded condition. Boost circuits may be most efficient, in terms of power lost within the circuit compared to power delivered to the load, when operating at almost peak capacity. In situations where multiple boost circuits are used, and the power to be supplied is significantly less than the rated capacity, efficiency of the switching power supply boost circuit may drop significantly. Moreover, off-the-shelf boost circuit controllers may not be capable of producing the multiple phase-shifted control signals to the boost switches in a multi-phase system.